FIELD OF THE INVENTION
This invention relates generally to the commercial refrigeration art, and more particularly to modular refrigeration system units strategically located in close proximity to product zones to be cooled and networked with an external condenser coolant system.
Although great advances have been made over the last 50 years in the design, convenience, operating efficiency and other aspects of refrigerated merchandisers and various commercial systems therefor, the conventional "remote machine room" approach to the location of system compressors has not changed. Of course, self-contained commercial cases, which like domestic refrigerators have their own condensing have always had a place in food merchandising, particularly in small convenience stores in which a few merchandising units can operate at relatively low noise levels. However, with the growth of retail food merchandising into large supermarkets, the expansion of commercial refrigeration requirements has been staggering. For example, a 50,000 square foot supermarket may have refrigerated display fixtures and other coolers and preparation rooms requiring an aggregate refrigeration capacity in excess of 80 tons (1,000,000 BTU/hr.) which may include over 20 tons of low temperature refrigeration at evaporator temperatures in the range of -35.degree. F. to -5.degree. F. and over 60 tons of normal temperature refrigeration at evaporator temperatures in the range of 15.degree. F. to 40.degree. F. Such presently existing commercial refrigeration systems have a multitude of evaporators (e.g., 100) for the various refrigerated product merchandisers located throughout the shopping area of the supermarket; and these evaporators are usually serviced by multiplexed low temperature and normal temperature compressor systems, each compressor typically being of the reciprocating type and located in the back room of the supermarket. It is not feasible to provide self-contained refrigerated product merchandisers (each with its own compressor) for stand-alone operation in a supermarket setting for numerous reasons, including cost and energy efficiency. Moreover, a single compressor in a self-contained case has no back-up in case of failure, no control over its rejected heat into the shopping arena, and a large number of reciprocating compressors would generate so much noise as to be totally unacceptable.
The most recent conventional practice is to put the massive refrigeration requirements of a supermarket into at least two multiplexed back room systems; one for the low temperature refrigeration of frozen foods and ice cream at product temperatures in the range of -20.degree. F. to 0.degree. F.; and another for the normal temperature refrigeration of fresh foods including meat, dairy and produce at product temperatures in the range of 28.degree. F. to 50.degree. F. Each such system is a closed system having a single condenser/receiver and liquid header with parallel circuits to the respective merchandiser or cooler evaporators and with the various complex valving requirements to balance suction pressures (EPR valves) and to accommodate selective evaporator isolation for hot gas or other types of defrosting. In any event, the multiplexed compressors of such systems are installed in remote or back machine rooms and typically connect to roof top air-cooled condensers, which in turn connect back to the machine room to a receiver and thence to the liquid header and various high side valving and liquid line circuit outlets. Again, the suction side of the various circuits are connected to a machine room suction header for each multiplexed system, and the various suction control EPR valves and hot gas distribution valves are located in this remote machine back room.
To connect the back room compressors and the store merchandiser evaporators for delivery and return of refrigerant in a large supermarket of the 50,000 square foot example, substantial lengths of refrigerant conduit piping must be employed, e.g., on the order of 18,000 feet of conduit may be required in which a large volume of relatively expensive refrigerant (e.g., 1800 pounds of Refrigerant 502 at about $8 per pound) is required just to fill these conduits for connection of the remote refrigeration systems. Should line breaks or leakage occur as from fissures in the conduits or joints (frequently caused by expansion and contraction of the conduits as during a defrost cycle), then substantial quantities of expensive refrigerant may be lost and the entire system jeopardized. The greater the length of the conduit, the more expansion will occur, creating a higher risk of breakage. It should also be recognized that, in response to environmental concerns over depletion of the ozone layer due to the release of various CFC products including different refrigerants, such as R-502 that have been commonly in use in the commercial supermarket refrigeration industry for many years, the government has imposed increasingly stricter limitations on such refrigerant usage. The result is that this industry, and others, are developing new non-CFC types of refrigerants as well as seeking other system arrangements and controls for minimizing environmental endangerment. However, such new refrigerants today are even more expensive than heretofore used in large volumes in typical prior art commercial system installations, thereby raising basic installation costs and creating higher loss risks in such conventional back room commercial systems. For instance, Refrigerant HP62, which is an HFC chemical, costs over $13 per pound.
So-called "cascade" refrigeration systems are well established refrigeration techniques where relatively low temperatures are to be achieved in the controlled zone or environment, particularly in industrial refrigeration and some cryogenic applications. In such cascade arrangements, a second stage is used to cool a first stage condenser. Briggs U.S. Pat. No. 3,590,595 discloses a cascade system for use with a remote primary system having a "back room" compressor/condenser arrangement with long liquid line conduits to the controlled refrigerated zone; and provides bypass means to obviate heat pickup and refrigerant vaporization due to intermittent evaporator cooling operations or other conditions in which the continuous liquid line flow to the evaporator is interrupted.
Perez U.S. Pat. No. 4,280,335 discloses an icebank refrigerating and cooling system utilizing off-peak ice storage as a direct primary refrigeration source for various supermarket normal temperature cooling purposes, such as air conditioning, produce, dairy and beverage cooling. Perez also suggests that the ice storage system can be employed as a cascade-type heat exchanger for another compressor/condenser system, but Perez discloses a only water loop from the return (heated) water conduit for this purpose. However, although thermal (ice) storage systems are prevalent in the refrigeration art, such technology is not considered practical as an alternative coolant source for commercial supermarket applications of the present invention for several reasons, among which is that the massive heat of rejection loads from the low and normal temperature merchandisers is carried by the return coolant circuit.